The present invention relates generally to gas shielded arc welding processes, and more particularly the invention relates to an improved gas shielded arc welding method well suited for the welding of the straight or helical seam of an open pipe for the manufacture of a very low temperature line pipe as well as the welding of other materials which must retain high toughness values at low temperatures.
Generally, large diameter pipe has been made by the UOE process, spiral process or the like on a mass production basis in a factory and therefore there has been a strong demand for a welding process of greater operating speeds and efficiency. Under these circumstances, the tandem sequence submerged arc welding process has been used as the process for the regular welding of the straight or helical seam of an open tube formed by the UOE or spiral process. This welding process may be generally considered as a welding process which is particularly adapted for the production of large diameter pipe since the process employs a large welding heat input which in turn results in a deep weld penetration and hence an excellent welding efficiency as compared with the other arc welding processes such as the inert gas metal arc welding process (hereinafter simply referred to as the MIG process) and the CO.sub.2 gas shielded arc welding process. Recently, there has been an increasing demand for the production of thick-wall (e.g. thicker than 25 mm) line pipe for low temperature applications. In contrast to the ordinary large diameter pipe, most of such line pipes are used for the purposes of conveying, under high pressured and at high speeds close to the velocity of sound, natural gases, etc. from very cold districts and consequently very high toughness at low temperatures must be ensured in the base metal and the weld zone. Moreover, there are many cases where the hardness of the weld zone (including the base metal) is limited to low values, namely, below 260 for Hv 1 Kg in order to prevent the occurrence of stress corrosion cracking due to the presence of sulfides. The above-mentioned tandem sequence large heat input submerged arc welding process may be advantageously utilized for the manufacture of such very low temperature line pipe to obtain satisfactory results in terms of welding efficiency. However, if this welding is used for the welding of the pipes with wall thickness of 25 mm, the welding is effected with a large heat input as high as 65000 Joule/cm with the result that a considerable deterioration of the properties, particularly the impact properties in the heat affected zone adjacent to the bond of the weld zone, takes place and this phenomenon is particularly noted in the properties of high quality steels such as low temperature steels. Therefore, the ordinary large heat input submerged arc welding process is not capable of ensuring the required properties in the welded materials of the above-mentioned type.
While, with the submerged arc welding process, the above-mentioned problem of deteriorated properties can be overcome only by limiting its welding heat input, the welding with such a low heat input ruins the characteristic features of the process and reduces its welding efficiency considerably, thus making it improper as the process for the manufacture of large diameter pipes. In other words, this makes it impossible to use the welding process which has heretofore been used customarily for the welding of large diameter pipes to provide the single layer, single pass welding on each of the inner and outer surfaces of a steel pipe and it is thus imperative to use the multi-layer welding which in turn results in a reduced welding efficiency. In addition, the multi-layer welding inevitably requires the flux removing operation after each pass and this also is directly reflected in the reduction of the welding efficiency. Another disadvantage is that to ensure the desired toughness at low temperatures, the submerged arc welding involves the use of a high basic flux independently of the welding heat input. Such a flux shows a high viscosity at elevated temperatures and essentially ill-suited for the high speed welding and it also frequently gives rise to welding defects such as the inclusion of the flux and slag.
In view of these deficiencies, the submerged arc welding process is not a well suited method for the welding of large diameter, thick wall open tubes for the manufacture of very low temperature line pipes, and therefore the use of the above-mentioned MIG welding process or the CO.sub.2 gas welding process may be considered as an alternative method. However, in these welding processes, the welding is usually accomplished in a gaseous atmosphere consisting principally of an inert gas such as argon or helium, or carbon dioxide gas by operating a small diameter wire of less than 2.4 mm.phi. with welding currents of less than 500 amp. With these welding processes, while it is possible to avoid the problem of deterioration of the properties in the weld zone by virtue of the reduced welding heat input, excepting the cases where the pieces to be welded are sheet steels, the maximum possible welding speed of these welding processes is up to 500 mm/min, namely, the welding efficiency is as low as two or three times that of the manual welding at the maximum. Consequently, the MIG welding process and the CO.sub.2 welding process cannot be adapted for any more than the tack welding of the seam at the best and these processes also fall short of the high efficiency welding processes adapted for mass production purposes. In an attempt to improve on such inefficiency, a welding method has been proposed in which four electrodes each consisting of a small diameter wire (1.6 mm.phi.) are arranged in a straight row along the weld line to accomplish the welding continuously. Even with this process, the welding efficiency still remains at a low level and thus it is impossible to turn the small current MIG welding process with small diameter wire into a practical method that can replace the tandem sequence, submerged arc welding process in terms of welding efficiency by merely increasing the number of electrodes used in the MIG welding process. Although a still another welding process adapted for the welding of very high tension steels and employing a large diameter solid wire of above 3.0 mm.phi. has been proposed, this process is also disadvantageous in that the maximum possible welding speed is limited to 250 mm/min with a resultant inefficiency and moreover the control of arc voltages is limited within 24 to 26 volts with a resultant instability of arc. Still further disadvantage is that this welding process takes the form of single electrode welding and therefore it also cannot be adapted for mass production purposes.